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Abstract

A system of two probes is designed for terahertz (THz) time-domain spectroscopy of sub-wavelength size objects. A twin-needle probe confines broadband THz pulses spatially by means of surface plasmon waves to a sub-wavelength spot smaller than 10 microns. The confined pulses are detected within the near-field zone of the twin-needle probe by a sub-wavelength aperture probe. The system allows THz spectroscopy to be applied to single micrometer-size objects in the 1-2.5THz region.

Figures (3)

Twin-needle probe system for THz spectroscopy. (a) Schematic diagram of the probe positioned above the aperture of the integrated near-field probe. The THz pulse is incident from above (indicated by red arrows). Inset: cross section of the system showing the needle tips, the aperture in the gold screen and the photoconductive antenna. (b) Optical image of the twin-needle probe. (c) Electric field distribution of the incident THz pulse (left) and the field distribution near the twin-needle probe (right). (d) Space-time maps of the incident THz pulse (left) and the pulse focused by the twin-needle probe (right). To show the field enhancement both space-time maps are normalized to the maximal amplitude for the twin-needle probe.

Confinement of the THz wave by the twin-needle probe. (a) High-resolution 100x100μm map of the electric field confined by the needles. (b) Linear scans of E2 along the x- and y-direction passing through the focus. (c) z-dependence of the time-integrated intensity of the confined THz field. Inset: waveform of the confined THz pulse. (d) Electric field distributions in the same area as in (a) at a larger distance z0 + Δz from the needle tips.

Spectral response of the twin-needle probe. (a) Schematic diagram of the experimental setup. THz pulses are generated in a ZnTe crystal, focused by a system consisting of a Si lens (L), two parabolic mirrors (P1, P4) and two flat mirrors (M2, M3) in the region of the needle probe (NP), and detected by the sub-wavelength aperture probe (AP). (b) Spectral response of the twin-needle probe (red squares and dotted line) and transmission spectrum of the sub-wavelength aperture (blue line). (c) Waveform and spectrum (inset) of the detected THz pulse.